General information on heat-related illness

Heat-related illnesses are caused by impaired dissipation of body heat. They occur:

• most commonly in hot environmental conditions, particularly if humidity is high
• if strenuous exertion generates excess metabolic heatLipman, 2019.

Heat-related illness ranges in severity from mild to life-threatening (heat stroke). In most forms, patients have hyperthermia—a core body temperature higher than 37.5°C. However, heat-related illness is not defined by body temperature alone; 15 to 56% of asymptomatic runners in several studies had core body temperatures of 40°C (others as high as 42°C); these people do not have heat-related illnessVeltmeijer, 2015. Clinical signs and symptoms are central to diagnosis of heat-related illness.

Milder forms of heat-related illness include heat rash, heat syncope (brief unconsciousness with no residual impairment of mental function) and heat (muscle) cramps (painful involuntary muscle spasms, usually after prolonged exercise and often in unacclimatised people, typically associated with sweating-induced loss of water and sodium). These mild illnesses occur in people whose temperature is generally below 40°C, but heat exhaustion and heat stroke must be excluded in these presentations. Symptoms usually respond to rest in a cool environment and rehydration.

Heat exhaustion is a mild to moderate form of heat-related illness, characterised by symptoms such as weakness, headache and dizziness. The key clinical feature of severe heat illness (heat stroke) is central nervous system dysfunction, presenting as an altered mental state or neurological signs, including seizures and comaLipman, 2019.